Automotive Mg & Al: Curb weight, $ and CO2
Transcript of Automotive Mg & Al: Curb weight, $ and CO2
Metall. Mater. Trans. A 38, (2007) 1649-1662 1/42
Carlos H. CáceresSchool of Engineering
The University of QueenslandBrisbane, Qld. Australia
Economic and Environmental Issues in Automotive Magnesium
Applications
Invited Lecture to ICAA-10 Vancouver, October 2006 Metall. Mater. Trans. A 38, (2007) 1649-1662.
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What drives the current interest in Al and Mg automotive applications?
Metall. Mater. Trans. A 38, (2007) 1649-1662 3/42http://www.innovaltec.com/iom3_dt/scamans_cans_to_lowco2_cars.pdf
Gasoline vehicles
Diesel vehicles
l/100 km
Weight Using Al and Mg lightens up the car and cuts gasoline consumption and emissions.
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Issues Regarding Automotive Applications
• What is the cost penalty of using Al and Mg in cars?
• Is the cost penalty related to the mechanical function?
• Is the gasoline saved by a lighter car enough to off-set the cost penalty of using light alloys?
• Does the gasoline saved by a lighter car off-set the environmental burden of producing Al and Mg?
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Light Alloy substitutions for Cast Iron and Steel in cars
• Same volume (Castings): Engine blocks, Valve covers
• Stiff Beams (bending): Steering wheels, Space frames
• Stiff Panels (bending) : Instrument panels, Door panels
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Material Indices and Exchange Constants
MF Ashby et al. 1992, 1996, 1999, 2003
Approach
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Material Indices for Minimum Mass
Function Index
Same Volume (castings)
Beam (bending)
Panel (bending)/ρE1/2
ρE1/3 /
1/ρ
minimise mass for given stiffness
minimise mass
Minimise mass?
Select a material that maximises this Material Index !
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How good are Al and Mg when it comes to reducing mass?
E(GPa)
ρ(Mg/m3)
Beam Panel Equal Volume
Steel 210 7.8 10 10
4.9
3.9
10
Al 75 2.7 5.9 3.5
Mg 44 1.7 5.1 2.2
ρ
3/1E
A 10 kgf component made of Steel…
ρ
2/1E
1ρ
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Material Indices for Minimum Mass
Function Index Same Volume (castings)
Beam (bending)
Panel (bending)
Material Indices for Minimum Cost?
cost [c ] = $/kg
minimise cost per unit vol
ρ/c E1/2
ρE1/3 /c
1/cρ
minimise cost for given stiffness
Want to minimise cost?
Use a material that maximises this Material Index !
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cheap car (Iron and Steel: cheap but heavy)
light car (Al, Mg: light but expensive)Conflicting
Goals
Designing with conflicting goals
Use trade-off plot(Ashby)
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P1 (mass)
B
P2
(cost)
Trade-off plot
1/2/Eρ
Plot all viable materials according to their material indices
Costfor givenstiffness
Massfor givenstiffnessParetto, 1906; Ashby, 2005
1/2c/Eρ
Materials substitution: join candidates by a
linear functionZ1
αSlope α = exchange constant
α is the cost penalty of substituting material B for A ($/kg)
What is the meaning of α ?
A
heavier
expe
nsiv
e
Mg
Fe
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Trade-off plot on log scales
Log scales
P1 (mass)
A
B
P2
(cost)
Linear scales
P1 (mass)
A
B
P2
(cost)
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•Cast Iron: about $0.5 US/kg
•Steel: about $0.8 US/kg
•Aluminium: about $2.5 US/kg
•Magnesium: about $3.4 US/kg
How much does it cost to put Mg or Al in a car ?
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How much does it cost to drive a car ? http://www.fuelgaugereport.com/U.S. Gasoline Fuel Price, September 2005
1 gallon = 3.78 lg => @ 3.1 $ per gallon => 0.8 $ per liter
2003
$US per gallon
month-year
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1 kg mass reduction, over 200 000 km vehicle lifeJohnson, 2002 ; IPAI, 2000
How much gasoline can we save per kg of mass reduction?
Driver’s (10 years) savings = 7 lg/kg = 6 $/kg
Curb weight and fuel economy
litres of gasoline
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CAFE Penalty: 0.5 $/kg
If a manufacturer does not meet the Corporate Average Fuel Economy [C.A.F.E.] standard, it is liable for a civil penalty of $5 for each 0.1 mpg (40 m/l) its fleet falls below the standard of 22.2 mpg
(9.4 km/l) (as of 2007).
What are the incentives for substituting Al or Mg for steel in automobiles?
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Manufacturer's upper bound: 0.5 $/kg (CAFE penalty)
Driver’s upper bound: 7 lg/kg = 6 $/kg (Driver's savings)
If the substitution costs you more than this, it is
not worth doing
When is a material substitution worth doing?
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Possible substitutions (2)
•Incumbent materials: Aluminium alloys
•Replaced by Magnesium alloys
Possible substitutions (1)
•Incumbent materials: Cast Iron and Steel
Replaced by
•Aluminium alloys
•Magnesium alloys
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0.1 1mass relative to steel beams
0.1
1
10
100
co
st re
lativ
e to
ste
el b
eam
s
AZ91
A356Steel Cast Fe
ρc/E
1/2
(103 $
m-3 G
Pa-1
/2)
0.1 1ρ/E1/2 (Mg m-3 GPa-1/2)
0.1
1
10
αAM = 7.6
αFA = 1.2
beams
αFM = 2.4
2
The cost penalty of Al or Mg Beams substitutions for steelMaterial
Costfor givenstiffness
Massfor givenstiffness
α FeAl= 1.2 $/kg
α FeMg = 2.4 $/kg
CAFE: α < 0.5 $/kg
Driver's savings
α < 6 $/kg
This is what a lighter vehicle will save you
(upper bounds)
1/2/Eρ
1/2c/Eρ
This is what it costs you to lighten up your vehicle ($/kg)
α AlMg = 9.9 $/kg
Metall. Mater. Trans. A 38, (2007) 1649-1662 20/420.1 1mass relative to cast iron
1 10ρ (Mg m-3)
1
10
100
ρc
(103 $
m-3)
AZ91 A356 SteelCast Fe
0.1
1
10
100
cost
rela
tive
to c
ast i
ron
αFA = 0.6
volume
αFM = 0.7
αAM = 1.3
2
Cost of Mg and Al Castings substitutions for cast iron
Costper m3
density
α FeAl= 0.6 $/kg
α FeMg= 0.7 $/kg
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1
10
α (
$/kg
)
Cast Fe-AlPanel Fe-Al
Beam Fe-Al
Cast Fe-Mg
Panel Fe-Mg
Beam Fe-Mg
Cast Al-Mg
Panel Al-Mg
Beam Al-Mg
CAFE penalty (0.5$/kg)
lifespan savings (6$/kg)
0.3
Driver's savings 6 $/kg
Cost analysis
Fe=>Al
Fe=>Mg
CAFE penalty 0.5 $/kg
α($/kg)
Substitutions below this line are OK for CAFE
Substitutions below this line are OK for Driver's
savingsAl=>Mg
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How about environmental (greenhouse gas, CO2)
effects?
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material h (kg of CO2 / kg)
Iron/Steel 1 ~ 2 kg/kg
Al ~12 kg/kg(45% hydro electricity, 55% fossil world
avge.)
~23 kg/kg (45% hydro electricity, 55% fossil world
avge.)
~42 kg/kg
Electrolytic Mg (30% of world production)
Pidgeon Mg (70% of world production)
Define: h = CO2 footprint: kg of CO2 per kg of alloy
Sources: IPAI (2000); Koltun et al. 2005; CES, 2006
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Function Index Same Volume (castings)
Beam (bending)
Panel (bending)
Material Indices to minimise CO2 creation?
CO2 footprint equivalent [hq ] = lg/kg
minimise CO2 per unit vol
qhρE /1/2
q /ρE h1/3
qhρ1/
minimise CO2 footprint for given stiffness
Maximise these !
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Gasoline equivalent to the CO2 footprint ?
Define: hq = (h / 2.85) lg/kg
Cars create ~ 2.85 kg of CO2 per litre of gasoline
hq = (equivalent) litres of gasoline burnt producing 1 kg of alloy
β = exchange constants involving CO2
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material hq
Iron/Steel ~ 0.5 lg/kgAl ~ 4 lg/kg
~ 8 lg/kg
~ 15 lg/kg
Electrolytic Mg
Pidgeon Mg
Sources: IPAI (2000); Koltun et al. 2005; CES, 2006
Gasoline equivalent to the CO2 footprint ?A lighter vehicle saves 7 lg/kg over 200x103 km
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CO2 creation: exchange constants for Same Volume substitutions (castings)
CO2footprint
mass
Electrolytic Mg
Driver's savings
β = 7 lg/kgPidgeon Mg
Al
gasoline burnt producing the materials to achieve one kg of mass reduction
This is what a lighter vehicle saves (per kg) over 2x105 km
ρ
ρhq
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Cast
Panel
Beam
Cast
Panel
Beam
Cast
Panel
Beam
Cast*
Panel*
Beam*
Cast*
Panel*Beam*
1
10
β (
lg/k
g)
CAFE liability(0.6 lg /kg)
βCO2
(7 lg /kg)
30
Al<MgFe<Al Fe<Mg Fe<Mg* Al<Mg*
CO2 creation analysis
Fe=>Al
Fe=>MgFe=>Mg*
Substitutions below the line are OK
Driver's savings 7 lg/kg
Al=>Mg
Al substitutions for Fe are OK
Electrolytic Mg substitutions for Fe: castings & panels
OK beams are off
Pidgeon Mg’s CO2-footprint is
excessive for beams and
panels, OK for castings
Electrolytic Mg substitutions for Al are viable for castings.
Pidgeon Mg is out of bounds.
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1 10 αCAFE (0.5 $/kg) α ($/kg) αS (6 $/kg)
1
10
β (l
g/kg
)
10 100break-even distance, dα (x103 km)
100
1000
brea
k-ev
en d
ista
nce,
dβ
(x10
3 km
)
βCO2
(7 lg /kg)
30
Simultaneous selection by Cost and CO2 footprint
β (lg/kg)
α ($/kg)CAFE liability
(0.5$/kg)
Driver's savings (7lg/kg)
Driver's savings (6
$/kg)
Distance to break-even (x103 km)
200x103 km
Substitutions inside the box are OK
Iron and steel replaced by Al
Iron and steel replaced by Mg
Iron and steel replaced by Pidgeon Mg
Aluminumreplaced by Mg
Aluminum replaced by Pidgeon Mg
•Only two substitutions are economically not viable.
• 8 out of 14 substitutions are environmentally not viable (primary alloys).
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Effect of recycling ?
• Recycling Al or Mg uses only about 5% of the energy required to produce primary metal.
• The exchange constants decrease in proportion to the recycled fraction.
Analysis so far assumed primary alloys
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Effect of recycling (post-consumers scrap)
• Al and Mg wrought alloys are nearly 100% refined metal. (Al: up to 8% is recycled metal)
• Al castings: as much as 60% is recycled metal.
• Diecast Mg : up to 20~ 35% is recycled metal.
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Effect of recycling on the driving distances to break even?
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Driving distances to break evenA cast Fe engine replaced by an Al or Mg engine
An engine block cast on a primary Al alloy (A356) requires 55x103 km to
break-even
Cast on alloy A319 (60% recycled) cuts the driving distance to ~10x103 km
Primary electrolytic Mg => 70x103 km
Primary Pidgeon Mg
=> 130x103 km
With 35% recycled Mg: Electrolytic Mg => 35x103 km Pidgeon Mg => 75x103 km
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Driving distance to break even for Al or Mg space frames replacing steel
A rolled Al panel requires
75x103 km to break-even
An extruded Al beam requires 130x103 km
to break-even
Wrought alloys are made of primary stock (Al: ~8% max old scrap), little benefit from recycling.
An extruded electrolytic Mg beam requires 210x103 km
to break-evenA rolled electrolytic Mg panel
requires 125x103 km to break-even
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Magnesium steering wheel replacing a steel steering wheel
Tzabari and Reich, 2000
A steering wheel of primaryelectrolytic Mg alloy requires 210x103 km to break-even
A steering wheel of primaryPidgeon Mg alloy requires
390x103 km to break-even
At 35% recycling rate electrolytic Mg requires
~130x103 km
At 35% recycling rate Pidgeon Mg requires
~260x103 km
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Al engine replaced by a Mg engine?
Al engine block cast on alloy A319 (60% recycled)
Primary electrolytic Mg => 323x103 km
Primary Pidgeon Mg
=> 724x103 km
With 35% recycled metal: electrolytic Mg => 160x103 km Pidgeon Mg => 430x103 km
Mg is not a good replacement for existing
cast Al components
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cars
Light trucks & USVAre current cars any lighter than back in 1970?
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Conclusions
• Cost and environmental penalties of light alloy applications strongly depend on the mechanical function.
• Penalty in decreasing order: castings, panels, beams.
• The cost penalty can be off-set by the savings of gasoline in most cases.
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CO2 - creation
• The high recyclability of Al casting alloys gives them a leading edge over both Al and Mg wrought alloys and Mg casting alloys.
• Pidgeon Mg is environmentally unsuitablefor most automotive applications.
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CO2-footprint according to the source of energy(Al and electrolytic Mg)
Energysource
100% hydro
/nuclear
55% fossil fuels 45% hydro/ nuclear
100% fossil fuels
Al 6.2 12 16.7Mg
(no SF6)7.5 20 30
Mg (with
SF6)10.6 23 33.1
present analysis
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Magnesium’s safest bet?
• E.U. imposing a tax on CO2 footprint should mark the end of Pidgeon Mg.
• Increased use of Hydro (or Nuclear Power) electricity to reduce Mg’s (and Al’s) CO2
footprint.
• Increasing the recycling rate of Mg castings.
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The End
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Extra slides:
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Titanium
β FeTi = 28 lg/kg
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CO2-footprint according to the source of energyAl and electrolytic Mg
present analysis: ~55% fossil fuels 100% fossil fuels
100% hydro/nuclear/other renewables
Clean sources of energy are essential for clean Mg or Al
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Green: α-values of the order of the CAFE liabilityBlue: within the savings over a Driver's of 2x105 km
Red: economically or environmentally not viableBrackets: distance to break even
Metrics Cost penalty($/kg)
Gasoline equivalent footprint (lg/kg) primary alloys
Function αFA αFM αAM βFA βFM β*FM βAM β*AM
Beam 1.2 (40)
2.2(73)
9.9 (330)
4.6 (132)
7.4 (212)
13.6 (389)*
25 (715)
76(2200)
*Panel 0.5
(17)1.1(37)
4.6 (153)
2.6 (74)
4.4 (126)
8.8 (252)*
12 (343)
40 (1144)
*Casting 0.4
(13)0.6(20)
1.3 (43)
1.9 (54)
2.3 (66)
4.7 (134)*
3.8 (109)
18 (514)*
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Critical Recycling Rates to make the exchange constants = 0 ?
volume panels beams
Al 82% 69% 75%
Mg 87% 82% 87%
Mg* 95% 93% 96%
Pidgeon Mg ~95%!
At these recycling rates Al and Mg create the same amount of
CO2/kg as Fe
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Transport System: mass saving α ($US per kg)
~0.5~6
5 to 20100 to 500
3000 to 10000
Family car (based on C.A.F.E. penalty)
Family car (based on Driver's savings)
Truck (based on payload)Civil aircraft (based on payload)Space vehicle (based on payload)
Finding α: Exchange Constants for Transport Systems
Ashby, 2005